1. Field of the Invention
The present invention relates to the field of electrical and optical cable, specifically to the in-situ monitoring of aging of various cable components.
2. Description of Related Technology
The aging of power, control, instrumentation, and data transmission cable is of considerable importance to, among others, industrial and electrical power plant operators in that the unanticipated failure of such cables may have significant adverse effects on plant operation and maintenance (O&M) costs and downtime. Electrical and optical cables have traditionally been considered long-lived components which merit little in the way of preventive maintenance or condition monitoring due to their generally high level of reliability and simplicity of construction. Like all other components, however, such cables age as the result of operational and environmental stressors. These stressors may include heat, mechanical stress, chemicals, moisture, nuclear radiation, and electrical stress. Furthermore, the presence of oxygen and contaminants, while not directly resulting in component degradation, may exacerbate the effects of the aforementioned stressors and accelerate the deterioration of various cable components. Aging effects may be spatially generalized (i.e., affecting most or all portions of a given cable equally, such as for a cable located completely within a single room of uniform temperature), or localized (i.e., affecting only very limited portions of a cable, such as in the case of a cable routed near a highly localized heat source). The severity of these aging effects depends on several factors including the severity of the stressor, the materials of construction and design of the cable, and the ambient environment surrounding the cable. Detailed discussions of electrical cable aging may be found in a number of publications including SAND96-0344 "Aging Management Guideline for Commercial Nuclear Power Plants--Electrical Cable and Terminations" prepared by Sandia National Laboratories/U.S. Department of Energy, September 1996. Discussions regarding optical cable aging may be found, inter alia, in Electric Power Research Institute (EPRI) publications and telecommunications industry literature. The following description will be limited to electrical cable, although it can be appreciated that the principles of aging and analysis described herein may also be largely applicable to optical cabling.
Electrical cables come in a wide variety of voltage ranges and configurations, depending on their anticipated uses. Cables can be generally categorized based on voltage range (i.e., low-voltage, operating below about 1000 Vac or 250 Vdc, medium-voltage, operating between about 2 and 15 kVac, and high-voltage operating above 15-kVac) or function (i.e., power, control or instrumentation). Existing prior art low- and medium-voltage power and control cables such as that shown in FIGS. 1 through 4 are typically constructed using a polymer or rubber dielectric insulation 10 which is applied over a multi-strand copper or aluminum conductor 20. The insulation is often overlayed with a protective polymer jacket 30. In multi-conductor cables (such as those used in three-phase alternating current systems, as shown in FIGS. 1 and 2), a plurality of these individually insulated conductors are encased within a protective outer jacket 40 along with other components such as filler 50 and drain wires (not shown). These other components fulfill a variety of functions including imparting mechanical stability and rigidity to the cable, shielding against electromagnetic interference, and allowing for the dissipation of accumulated electrostatic charge. This general arrangement is used for its relatively low cost, ease of handling and installation, comparatively small physical dimensions, and protection against environmental stressors.
Current methods of evaluating electrical cable component aging generally may be categorized as electrical, physical, and microphysical. Electrical techniques involve the measurement of one or more electrical parameters relating to the operation of the cable, such as the breakdown voltage, power factor, capacitance, or electrical resistance of the dielectric. These methods have to the present been considered largely ineffective or impractical, in that they either do not show a good correlation between the parameter being measured and the aging of the dielectric, or are difficult to implement under normal operations. Furthermore, such techniques are often deleterious to the longevity of the insulation, and have difficulty determining localized aging within a given conductor.
Physical techniques including the measurement of compressive modulus, torsional modulus, or rigidity under bending often show a better correlation between the aging of the cable and the measured parameter (especially for low-voltage cable), and are more practical to apply during operational conditions. However, they generally suffer from a lack of access to the most critical elements of the cable, the individual electrical conductors and their insulation. For example, the measurement of compressive modulus by way of instruments such as the Indenter Polymer Aging Monitor are effective primarily with respect to the outer, accessible surface of the cable such as its outer jacket. Although correlations of the aging of the outer jacket to that of the underlying conductors have been attempted, these correlations are generally quite imprecise and are subject to a large degree of variability based on the specific configuration of the cable being tested (i.e., its materials of construction, insulation/jacket thickness, etc.), the presence of ohmically induced heating, shielding of the conductors against stressors by the outer jacket, and differences in the oxygen concentration at the conductor insulation versus that at the outer jacket. See EPRI TR-104075, "Evaluation of Cable Polymer Aging Through Indenter Testing of In-Plant and Laboratory Aged Specimens," prepared by the Electric Power Research Institute, January, 1996 for a discussion of the correlation between outer jacket and conductor physical measurements.
Other physical techniques such as the measurement of the tensile strength or elongation-at-break of the insulation material are inherently destructive and require a specimen of the aged cable for testing.
Another potential drawback to many of the physical techniques described above is disturbance of the bulk cable run during testing. In some applications, the dielectric of the cable being evaluated may be highly aged and embrittled, yet still completely functional. However, substantial movement of the cable (such as picking the cable up and clamping on a test device) may produce elongation stresses beyond those corresponding to the elongation-at-break for the insulation and/or jacket material, thereby inducing unwanted cracking of the insulation and/or jacketing and potential electrical failure.
Microphysical techniques such as the measurement of insulation oxidation induction time (OIT), density, gel or plasticizer content, or infrared absorption spectrum are generally quite accurate, yet require samples of the cable insulation and/or jacket for analysis. For jacketed conductors, such samples are generally only available at the ends of the cable where the conductors are terminated to a source or load, and not anywhere between. Furthermore, as with the physical techniques described above, the results of any such testing are necessarily applicable only to the localized area of the cable from which the specimen was taken, which may or may not be representative of the rest of the cable. Hence, one can either take a small sample of material from the outer jacket of the cable and attempt to extrapolate the results of the aging analysis to the underlying conductors, or alternatively take a sample at the ends of the conductor itself near its terminations and extrapolate these results to the rest of the unexposed conductor. Under either alternative, a substantial degree of uncertainty and imprecision exists. Plant operators are also generally reticent to allowing the removal of even small samples of material from their cables, especially in applications where plant safety and continuity of electrical power are critical.
Based on the foregoing, it would be most desirable to provide a cable construction which allows an operator to more accurately assess the aging of the underlying conductor insulation and jacketing of an in-situ cable (or optical conductor in the case of a fiber optic cable) in a completely non-destructive manner.
Similarly, it would be desirable to provide a method of monitoring and estimating the aging of internal cable components using the aforementioned cable construction in conjunction with aging measurement techniques and protocols.